Further characterization and purification of a circulating antigen in schistosomiasis revealed that the material is a large molecular weight polysaccharide composed primarily of D-glucuronic acid and N-acetylgalactosamine. Specific IgM and IgG antibody responses to this antigen can be measured by indirect immunofluorescent techniques. Both IgG and IgM titers are highest in acute infections and gradually fall to low titers in chronically infected patients. The fall in titer is dependent only on the duration of infection and occurs despite continued intense antigenic stimulation. As a diagnostic test specific IgM antibody is found in 100% of active cases; the false positive rate is 3%.